Miniaturization of analytical systems has played an important role in the development of fast analysis systems using very small amounts of sample. During the last decade, intensive efforts have been devoted to the design of whole laboratory systems on micro-analytical substrates as analytical tools, utilising for example, capillary electrophoresis and liquid chromatography. However, developments in microanalysis have been hindered to some extent by the lack of efficient fluid pumping systems. Fluid movement in micro-channel networks formed in glass, silica or polymeric substrates is conventionally driven by capillary flow or electro-osmotic pumping. The former process relies on chemical modifications of the surface properties of the micro-analytical substrate, which in turn establish restrictions on the flow direction and rate. The latter process involves high-voltage inputs through electrodes in contact with a solution, which is usually referred to as electrokinetic pumping.
A critical parameter in performing high-resolution electrophoresis or electrochromatography is the injection. In standard capillary electrophoresis two injection methods are generally used. The first method, electrokinetic injection, is easy to perform and requires placing the capillary in contact with a sample solution and pumping the latter inside the capillary by applying a high voltage. However, this injection method is not always suitable as in the case where some of the species in the sample to be analysed have very different electrophoretic mobilities. In this case some of the species will migrate during the injection, resulting in bias in the final concentration ratio between the analytes. For instance, the injection of a sample in pure water in order to generate a strong stacking effect is very difficult using this method. Also, the injection of very complex mixtures, for example samples of high protein concentration, is difficult, because the proteins may change the zeta potential in the sample channel resulting in instability in the flow intensity or even in the electro-osmotic flow direction. Several injection patterns based on electrically driven flow have been proposed such as T-injection, double-T-injection as well as electrically pinched injection as exemplified in U.S. Pat. Nos. 5,858,195; 6,001,229; 6,010,607; 6,010,608; and 6,033,546. All of these inventions were linked to the application of a high voltage facilitating pumping of the sample solution. Several potentials can be applied in different channels of the device in order to force the solution to flow producing well defined sample plugs. The principal drawback of the inventions described is the necessity to work with samples which have well-defined conductivity, viscosity and thermal properties. Therefore, analyses of other samples has to be performed using the second method of injection, pressure driven injection, which is less sensitive to sample composition.